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Probably every portrait photographer resorts to some such "dodge" for making a face lighter or other similar purpose, and he finds that the application of water-color paint in ever so thin a wash makes so much difference to the printing density that blue is commonly used as being a quick printing color, the effect of which is therefore not so great; an equally dense layer of blue and of red to any portion of a negative would print very differently, the red shutting off the light action much more powerfully than the blue.
— ALFRED HORSLEY HINTON